The present invention relates to a reinforcing steel having distinguished resistance to salt and preventive of deterioration of concrete, suitable for use in concrete structures or concrete bridges which are built on beach, seashore or offshore where they are exposed to salt particles or splashes of sea water.
Recently, various methods have been proposed and carried out which are preventive of cracking in various steel-reinforced concrete building construction making use of sea sand, as well as concrete structures which are situated on beaches or seashores.
The deterioration of concrete in the constructions and structures of the type mentioned above is attributable mainly to corrosion of the reinforcing steel by salt inherently contained by the sea sand or salt which has penetrated the concrete wall which is subjected to the salty atmosphere in the shore area. Namely, the corroded steel expands to increase its volume to about 2.2 times the original volume through the creation of corrosion products, so that the concrete can not withstand the expansion to thereby cause cracks along the reinforcing steel. When the crack grows to 0.2 mm or greater in size, oxygen, salt and carbon dioxide gas in the air penetrates into the concrete through the crack to reach the reinforcing steel. The salt promotes both the corrosion of the reinforcing steel and neutralization of concrete, thus accelerating the deterioration of the concrete.
The present inventors have made an intense study for developing reinforcing steel having improved salt resistance, through a suitable control of the chemical composition of the reinforcing steel and addition of special elements to the steel composition, and succeeding in developing a reinforcing steel as shown in Japanese Unexamined Patent Publication Nos. 48054/1982 and 44457/1984. This steel is disclosed also in other literatures such as "OFFSHORE GOTEBORG '81" Paper No. 42, Goteborg SWEDEN 1981, "CEMENT CONCRETE" No. 434 (1983) P. 23/31, and "Corrosion of Reinforcement in Concrete Construction" P. 419, 1983. These literatures detail the anti-salt mechanisms of the elements of the steel which contributes to the improvement in the anti-salt properties, and some of the steel compositions proposed in these literatures have been already put into practical use.
It is a current demand to substantially prevent the corrosion of the reinforcing steel and the resultant cracking in the concrete, both of which are attributable to penetration of salt particles and the splash of salt through concrete walls.
Problems are becoming serious in various fields in regard to cracking in concrete constructions and structures which are 10 or more years old. This is because the salt content of the concrete around the embedded reinforcing steel reaches 0.3 to 0.5 wt% when calculated in terms of NaCl amount in the sand of concrete, and has caused heavy corrosion of the reinforcing steel, resulting in crack occurrence and growth. Concrete buildings and structures of 30 or more years old often exhibit a high salt content exceeding 0.5 wt% when calculated in terms of NaCl amount in the sand of concrete.
It is, therefore, highly desirable to develop a technique which can prevent cracking in the concrete even when the salt content is as high as 0.5 wt% in terms of Nacl amount in the sand of concrete, and also to make it possible to remarkably retard the cracking in the concrete even when the salt content exceeds 0.5 wt% in terms of Nacl amount in the sand of concrete.